1. Field of the Invention
The present invention relates to a jaw-advancing mechanism for a fixture.
2. Description of Prior Art
A fixture is a clamping tool for fixedly clamping an object. For example, several pieces of wood plates are clamped by a fixture, whereby the wood plates can be bonded together or processed by a machine simultaneously.
FIG. 1 shows a conventional fixture 10 having an elongate rod 11, a first jaw 20 and a second jaw 30, which is used in the field of furniture or woodworking. Both ends of the elongated rod 11 are formed with a post 12 to restrict the length of one end of the elongated rod 11 inserted into the first jaw 20. A positioning hole 13 is formed on the elongated rod 11 between its one end and the post 12. A fixing knob 21 passing through the first jaw 20 is disposed in the positioning hole 13, whereby the first jaw 20 can be fixed to one end of the elongated rod 11. If necessary, the first jaw 20 may be mounted to another end of the elongated rod 11.
The second jaw 30 is integrally formed with a pivotal portion 31 and a grip 40 by means of an injection molding process. The elongated rod 11 is disposed through the pivotal portion 31, so that the second jaw 30 and the grip 40 can reciprocate synchronously between the two posts 12 to thereby change the relative distance between the first jaw 20 and the second jaw 30. A trigger 50 is provided opposite to the grip 40. The trigger 50 has a forked end straddling the pivotal portion 31. The forked end is formed with a supporting portion 51. An edge of the supporting portion 51 is formed into a semi-circular surface for matching a curved wall 52 formed inside the pivotal portion 31 and supporting the trigger 50 to swing relative to the grip 40. Further, a push rod 53 is provided at the forked end of the trigger 50 to pass through the pivotal portion 31. The push rod 53 swings synchronously with the trigger 50 and prevents the trigger 50 from leaving the pivotal portion 31.
The interior of the pivotal portion 31 is divided into two space including a front chamber 32 and a rear chamber 33. The front chamber 32 is a space defining a region adjacent to the curved wall 52, and the rear chamber 33 is a space adjacent to the front chamber 32. The elongated rod 11 is disposed through the front chamber 32 and the rear chamber 33. Three clasping pieces 34 and a restoring spring 35 are received in the front chamber 32. The middle portion of each clasping piece 34 is provided with a rectangular through-hole 39 for allowing the elongated rod 11 to pass through. The restoring spring 35 abuts against an inner wall of the front chamber 32 to thereby push the clasping pieces 34 to advance in such a manner that the clasping pieces 34 are substantially kept perpendicular to the elongated rod 11. In this way, the inner edge of the rectangular hole 39 will not rub against the elongated rod 11. Further, the lower edge of the clasping piece 34 abuts against the push rod 35, thereby causing the trigger 50 to swing in the same direction.
A triggering piece 36, a damping piece 37 and a compression spring 38 are received in the rear chamber 33. The triggering piece 36 is brought into contact with the damping piece 37. The elongated rod 11 is disposed through the triggering piece 36, the damping piece 37 and the compression spring 38. The compression spring 38 abuts against an inner wall of the rear chamber 33 and the damping piece 37 to push the triggering piece 36 to be inclined with respect to the elongated rod 11. As a result, the second jaw 30 can stop on the elongated rod 11. A release button 41 is provided on an outer surface of the grip 40 for driving the triggering piece 36. When a user presses the release button 41 by his/her finger, the release button 41 drives the triggering piece 36 and the damping piece 37 to swing in a direction perpendicular to the elongated rod 11, thereby releasing the second jaw 30. As a result, the second jaw 30 can advance along the elongated rod 11 to press the compression spring 38.
As shown in FIG. 2, when an external force F is applied to the trigger 50 to cause the trigger 50 to swing toward the grip 40 by using the supporting portion 51 as a center of rotation, the push rod 53 pushes the clasping pieces 34 to get an inclined position relative to the elongated rod 11. At this time, the outer periphery of the elongated rod 11 touches the inner edges of the rectangular holes 39 of the clasping pieces 34, thereby generating a reaction moment to prevent the clasping pieces 34 from moving relative to the elongated rod 11. Since the external force F is larger than the stopping force caused by the clasping pieces 37 on the elongated rod 11, the push rod 53 of the trigger 50 can abut against the lower edges of the clasping pieces 34 to swing continuously, thereby driving the second jaw 30 to move a certain distance in the direction of the arrow D. Thus, the clasping pieces 34, the restoring spring 35 and the trigger 50 constitute an advancing mechanism to help the second jaw 30 to move on the elongated rod 11.
Please refer to FIG. 3. When the external force is released, the trigger 50 cannot oppose against the stopping force caused by the clasping pieces 37 on the elongated rod 11, which causes the second jaw 30 to stop on the elongated rod 11 at a new position. At this time, the clasping pieces 34 are pushed by the elastic force of the restoring spring 35 to counteract with the reaction moment applied on the elongated rod 11. As a result, the clamping forces 37 move in the direction of the arrow d, whereby the push rod 53 drives the trigger 50 to swing outwards around the supporting portion 51 away from the grip 40.
However, such a conventional fixture has a problem that the second jaw 30 may be unable to advance sometimes. The reason is as follows. As shown in FIG. 2, the restoring spring 35 is a wound metallic coil. The winding of the coil (especially its end) contacting the clasping piece 34 may overlap on a region adjacent to the outer periphery or inner diameter of the next winding of the coil, which impedes the extension of the restoring spring 35. As a result, the clasping pieces 34 no longer tightly contact the push rod 53, so that the trigger 50 is loosened and unable to drive the second jaw 30 to advance continuously. At this time, the fixture cannot be used unless the user pulls the trigger 50 for several times until the coil of the restoring spring 35 returns to its original shape. Therefore, it is an important issue to solve this problem.
In view of the above, the present inventor proposes a novel and improved fixture based on his expert experience and delicate researches.